In the simplest form, an organic electroluminescent (EL) device is comprised of an organic electroluminescent media disposed between first and second electrodes serving as an anode for hole injection and a cathode for electron injection. The organic electroluminescent media supports recombination of holes and electrons that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. A basic organic EL element is described in U.S. Pat. No. 4,356,429. In order to construct a pixelated OLED display device that is useful as a display such as, for example, a television, computer monitor, cell phone display, or digital camera display, individual organic EL elements can be arranged as pixels in a matrix pattern. These pixels can all be made to emit the same color, thereby producing a monochromatic display, or they can be made to produce multiple colors such as a three-pixel red, green, blue (RGB) display.
OLED display devices have been fabricated with active matrix (AM) driving circuitry in order to produce high performance displays. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066. However, in this type of display device, when light is emitted downward through the substrate, the overall area that can emit light is limited by the presence of thin film transistors (TFT's) and other circuitry, which are opaque. The area of the display pixels that emits light relative to the total area of the pixels is known as the aperture ratio (AR) and is typically less than 50% in such displays. In order to compensate for lower AR, the device must be driven at a higher current density compared to a device with a high AR. The result is that the lower AR devices use more power and have a shorter useable life than a device with a higher AR.
Therefore, much work has been done to produce AM OLED display devices that are top-emitting (or surface-emitting), that is, where light is removed through the upper surface away from the substrate and active matrix circuitry. Such a display device is described in U.S. Pat. No. 6,737,800. This top-emitting configuration allows for increased AR and therefore improved performance of the display.
One approach to forming a multi-color display device involves the use of a broadband light-emitting EL structure, such as a white light-emitting EL structure coupled with a multi-colored array of color filter elements. In this configuration, a single organic EL layer structure can be applied to all pixels, and the color perceived by the viewer is dictated by that pixel's corresponding color filter element. Therefore, a multicolor or RGB device can be produced without requiring any patterning of the organic EL layers. Such white-light top-emitting AM display devices with CFA's can offer superior AR, manufacturing yield, and production throughput compared to top-emitting AM display devices with multicolor patterning. An example of a white CFA top-emitting display device is shown in U.S. Pat. No. 6,392,340.
Multi-color OLED display devices have also recently been described that are constructed with four differently colored pixels. One example of such a four pixel display includes pixels that are red, green, blue, and white in color. This configuration is known as an RGBW type display. Examples of such four pixel displays are shown in U.S. Pat. Nos. 6,771,028, 7,012,588, U.S. Patent Application Publications 2004/0113875 A1, and 2004/0201558 A1. Such RGBW displays can be constructed using a white organic EL emitting layer with red, green, and blue color filters for the red, green, and blue pixels, respectively. The white pixel area is left unfiltered. Inclusion of the unfiltered white pixel allows for the display of colors that are less than fully saturated at reduced power consumption compared to similar RGB only, filtered broadband OLED displays.
When manufacturing organic EL displays, problems such a particle contamination or scratches in the organic EL materials can result in defects in a display. One type of defect that is caused by particle contamination or scratches is a short circuit through the thin organic materials, connecting the anode and the cathode. A short between the anode and cathode results in a non-emitting pixel (dead pixel) or a pixel that emits at reduced brightness (dim pixel).
Similarly, defects due to particles, scratches, mask-errors, electrostatic discharge (ESD), and similar manufacturing problems can occur in the manufacturing of the active matrix circuitry of an active matrix OLED display device which results in non-functional pixels or pixels which emit at an incorrect luminance level (dim or bright). Alternately, defects in the active matrix circuitry can result in a condition where a pixel continuously emits during operation and can not be selectively turned off. This is type of defective pixel is referred to as a stuck-on pixel.
Many of these manufacturing defects, typically occur with an area density that depends on the capability of the manufacturing process, equipment and the environment. However, the total yield with respect to non-emitting pixels depends on the area density of the defects and the area of the individual displays. Larger display devices, such as those useful for televisions, computer monitors, or laptops, will have lower yields than smaller substrates given the same defect density.
In a multicolored device, a dead, dim, bright, or stuck-on pixel in one color can cause color distortions in the image. That is, multiple different color pixels such as the red pixel and the green pixel, for example, can be simultaneously illuminated in a predetermined luminance ratio so that the viewer perceives a desired color, such as yellow. With this example, if one of the pixels, such as the green pixel, is dead or dim, the resulting image will not appear the desired yellow color in the area of the defective pixel but will instead appear to be more red in color.